The present invention generally relates to a power feeding apparatus which is used in a high-frequency heater or the like, is further power-converted by a transformer after the power provided by a power supply such as a commercial power supply or the like has been converted into the high-frequency power by a transducer including a semiconductor, and feeds the converted power into the load having the unidirectional electrical current characteristics of a magnetron or the like.
Generally, such a power feeding apparatus is provided in various constructions with the views of smaller size, lighter weight, lower cost, etc. of chiefly the apparatus transformer. A magnetron is a representation in the example of the load having unidirectional electrical current characteristics and requiring comparatively large power. Many improvements are provided, even with respect to such a magnetron and the propositions thereof are provided.
FIG. 1 shows one embodiment of the conventional power feeding apparatus of this type. It is the same in construction as that shown in Japanese Patent Application Publication (unexamined) Tokkaisho No. 259388/1986.
With the construction of FIG. 4, operate a transistor 6 with the frequency of approximately 20 KHz through 100 KHz, and the weight and size of the boosting transformer may be made from one severalth to one ten-oddth as compared with the boosting with the commercial power-supply frequency being as it is.
The power of a commercial power supply 9 is rectified by a diode bridge 10 to form a unilateral power supply. The commercial power supply 9 and a DC power supply circuit 13 form the power supply of an inverter 14. It is to be noted that a choke coil 11 and a smoothing capacitor 12 play the role of a filter with respect to the high-frequency switching operation of the inverter 14.
The inverter 14 is composed of a resonance capacitor 5, a transformer 2, the transistor 6, a diode 7 and a driving circuit 8. The transistor 6 effects the switching operation with the given period and duty (on, off time ratio) by the base current to be fed from the driving circuit 8. As a result, a current with such a collector current I.sub.CE and a diode Id as shown in FIG. 5(a) as a center flows to the primary winding of the transformer 2. Such a current I.sub.L as shown in FIG. 5(b) flows.
A load 3 having a unidirectional electrical current characteristic is connected onto the secondary side of the transformer 2. The power converted by the inverter 4 is to be fed to the load 3. The load 3 is equivalently one like a magnetron represented by a series connector of a diode D.sub.M, a resistor R.sub.M, and a zener diode Z.sub.DM.
Such a current I.sub.A as shown in FIG. 2(b) flows to the load 3. The voltage V.sub.AK of the load 3 becomes as shown in FIG. 2(a). This is because the transformer 2 is a leakage type transformer, further a capacitor CH4 which a reverse bias current bypass means is connected in parallel relation to the load. Namely, the load 3 may be replaced by a series circuit of a resistor R.sub.M, a diode D.sub.M, and a zener diode Z.sub.DM. The capacitor CH is connected in parallel to it.
The load 3 is non-linear. The impedance (almost open) becomes very large through the diode D.sub.M with respect to the reverse voltage (normal-direction voltage). On the other hand, the impedance becomes large before a certain constant voltage (zener voltage of Z.sub.DM) is exceeded with respect to the positive voltage (negative direction voltage). The impedance becomes small when this voltage is exceeded. The magnetron is such a load as described hereinabove. The characteristics are shown in FIG. 3.
In FIG. 2(a), the load conducts when the voltage V.sub.AK of the load 3 is -4 KV. If the voltage on the primary side rises because of the low impedance, approximately -4 KV is retained. Also, at this time, the load current IA flows.
When the reverse voltage is applied, the load becomes very high in impedance. Thus, such a voltage as shown in FIG. 2(a) is caused through the connection with the capacitor CH4 for the bypass use of the reverse bias. The voltage is approximately 10 KV. It may be somewhat smaller by the larger capacitor CH4. The charging current into the capacitor CH4 increases correspondingly. The copper loss caused in the winding of the transformer becomes large, with the temperature rise is caused through the heating. In approximately optimum capacitor capacity, the reverse voltage becomes approximately 10 KV.
In such a power feeding apparatus, the voltage V.sub.AK of approximately 10 KV is applied, so that the insulation failure of the corona discharge, the arc discharge or the like is caused from between both the terminals of the load 3 to destroy the apparatus. As this voltage is made smaller, it is lowered somewhat by the larger size of the capacitor CH4 for bypass use of the reverse bias. The charging current of the capacitor becomes larger correspondingly. The copper loss of the transformer 2 increases to perform the heating operation so that the transformer fails because of the insulating destruction. It is very difficult to prevent the corona discharge or the arc discharge through the lowered voltage V.sub.AK like this.